An intravenous membrane oxygenator(IMO) is being developed by the Universityof Pittsburgh's Artificial Lung Program. The IMO is intended to be a temporary assist device for critically ill patients with Adult Respiratory Distress Syndrome. Oxygen is delivered to the blood and carbon dioxide is removed from it by diffusion across microporous hollow membrane fibers. A buindle of these fibers, surrounding a central balloon, is placed in the vena cava, the large vessel that returns blood to the heart. The balloon is pulsed rhythmically in order to enhance convective mixing around the fiber membranes and thus promote more efficient gas exchange. Initial in vitro experiments demonstrate that gas exchange varies with the balloon pulsation rate. We believe that balloon size and geometry are key design parameters in the optimization of IMO performance. A computational model has been developed to examine the velocity field produced by balloon pumping in an idealized vana cava. We propose to use this model to study several design variables for the IMO's component balloon. The results of the proposed study will serve as input to the next generation of prototypes.